Hydrogen Effects on Air Oxidation of Zirlo Alloy

NUREG/CR–6851
ANL-04/14
Hydrogen Effects on
Air Oxidation of
Zirlo Alloy
Argonne National Laboratory
U.S. Nuclear Regulatory Commission
Office of Nuclear Regulatory Research
Washington, DC 20555-0001
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NUREG/CR-6851
ANL-04/14
Hydrogen Effects on
Air Oxidation of
Zirlo Alloy
Manuscript Completed: August 2004
Date Published: October 2004
Prepared by
K. Natesan, W.K. Soppet
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439
S. Basu, NRC Project Manager
Prepared for
Division of Sytems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code Y6512
ii
Hydrogen Effects on Air Oxidation of Zirlo Alloy
by
K. Natesan and W. K. Soppet
Abstract
An experimental program was conducted to generate data on the air oxidation kinetics of
unirradiated Zirlo cladding with preoxidation and prehydriding to simulate inventory of spent fuel
discharge after a medium or high level of fuel burnup. The oxide layer on the cladding was
formed by a preoxidation step in a steam environment for 140 h at 550°C, which resulted in an
oxide thickness in the range of 25-30 µm. Prehydriding treatment was done by charging
hydrogen to cladding and the process was tailored to produce hydrogen concentration in the
range of 100-1000 wppm, typical of medium to high burnup cladding. The prehydrided and
prehydrided/steam-preoxidized specimens were subsequently oxidized in air at temperatures in
the range of 300-600°C. The maximum air oxidation times ranged between 300 h at 600°C
and ≈1000 h at 300°C. Weight-change and oxide-thickness were measured on the specimens
exposed at various times to establish the kinetics of the scaling process as a function of
temperature. Extensive metallography and hardness measurements were performed on the
tested specimens to examine the oxide scale development and hydrogen ingress into the
material. Weight-change and oxide-thickness data, generated in the present program, were
used to develop correlations to depict the air oxidation behavior of prehydrided alloys with and
without steam preoxidation. A comparison of the oxidation data on Zirlo with and without
prehydriding (performed in gas phase hydrogen and/or in steam) indicated that hydrogen
concentration in the range of 100-1000 wppm had minimal effect on the Zirlo oxidation rate in air
at temperatures in the range of 300-600°C.
iii
iv
Contents
Abstract ....................................................................................................................................
iii
Executive Summary .................................................................................................................
ix
Foreword ..................................................................................................................................
xi
Acknowledgments....................................................................................................................
xiii
1
Introduction......................................................................................................................
1
2
Background .....................................................................................................................
3
3
Experimental Procedure .................................................................................................
5
3.1
Material................................................................................................................
5
3.2
Specimen Geometry ...........................................................................................
5
3.3
Prehydriding of Ring and Capsule Specimens...................................................
5
3.4
Steam Exposure of Prehydrided Ring and Capsule Specimens .......................
8
3.5
Air Exposure........................................................................................................
8
Prehydriding of Zirlo........................................................................................................
11
4.1
Gas Phase Hydriding..........................................................................................
11
4.2
Hydriding in Steam..............................................................................................
12
4.3
Hydriding of Capsules.........................................................................................
13
5
Air Oxidation of Prehydrided Zirlo...................................................................................
15
6
Air Oxidation of Prehydrided and Steam-Preoxidized Zirlo............................................
19
7
Correlations for Oxidation Kinetics .................................................................................
25
8
Conclusion.......................................................................................................................
29
References...............................................................................................................................
31
4
v
Figures
2.1
Zr-H phase diagram, indicating extremely low value for hydrogen solubility in Zr
alloy at the reactor operating temperature of 320°C .................................................
4
2.2
Solubility of hydrogen in Zircaloy-4 as a function of reciprocal temperature ............
4
2.3
Total hydrogen concentration as a function of volume fraction of ZrH2 phase at
temperatures in the range of 100-600°C: 0-0.1 ZrH 2 and 0-0.01 ZrH2 .....................
4
3.1
Test facility used for hydrogen charging of ring and capsule specimens of Zirlo.....
6
3.2
Typical ring specimens of Zirlo after exposure for hydriding at 320°C .....................
7
3.3
Capsule specimens of Zirlo before and after hydriding treatment... .........................
7
3.4
Test facility used for steam preoxidation of prehydrided Zirlo specimens................
8
4.1
Hydrogen concentration in Zirlo ring specimens as a function of exposure time in
pure hydrogen gas at 320°C......................................................................................
11
Hydrogen concentration and ZrO2 thickness in Zirlo ring specimens as a function
of exposure time in steam at 550°C ..........................................................................
12
Hydrogen concentration as a function of oxide thickness for Zirlo specimens
steam oxidized at 550°C............................................................................................
13
Weight change in 75-mm-long capsule specimens after 170-h exposure in pure
hydrogen gas at 320°C ..............................................................................................
14
Weight change during air oxidation in capsule specimens of bare and
prehydrided Zirlo after 170-h exposure in pure hydrogen gas at 320°C...................
16
Weight change during air oxidation in capsule specimens of bare Zirlo specimens
and in ring specimens of prehydrided Zirlo that were exposed for 452 h in pure
hydrogen gas at 320°C ..............................................................................................
17
SEM photomicrograph of cross section of Zirlo capsule prehydrided for 170 h at
320°C .........................................................................................................................
17
SEM photomicrograph of cross section of initially prehydrided Zirlo capsule, after
air oxidation for 500 h at 500°C. ................................................................................
18
SEM photomicrograph of cross section of initially prehydrided Zirlo capsule, after
air oxidation for 316 h at 600°C .................................................................................
18
Weight change data obtained at 500 and 600°C during air oxidation of Zirlo
capsule specimens that were initially either only steam oxidized or
prehydrided/steam oxidized.......................................................................................
20
Weight change data obtained at 300 and 400°C during air oxidation of Zirlo
capsule specimens that were initially either only steam oxidized or
prehydrided/steam oxidized.......................................................................................
20
Oxide thickness data obtained at 500 and 600°C during air oxidation of Zirlo
capsule specimens that were initially either only steam oxidized or
prehydrided/steam oxidized.......................................................................................
21
Oxide thickness data obtained at 300 and 400°C during air oxidation of Zirlo
capsule specimens that were initially either only steam oxidized or
prehydrided/steam oxidized.......................................................................................
21
4.2
4.3
4.4
5.1
5.2
5.3
5.4
5.5
6.1
6.2
6.3
6.4
vi
6.5
Knoop microhardness indentations on as-received Zirlo cladding tube ...................
22
6.6
Knoop microhardness indentations on initially prehydrided Zirlo cladding tube
after 1026-h air oxidation at 300°C............................................................................
22
Knoop microhardness indentations on initially steam-oxidized Zirlo cladding tube
after 1000-h air oxidation at 300°C............................................................................
22
Knoop microhardness indentations on initially prehydrided/steam-oxidized Zirlo
cladding tube after 973-h air oxidation at 300°C.......................................................
22
6.9
Knoop hardness profiles for as-received Zirlo in four quadrants ..............................
22
6.10
Knoop hardness profiles in four quadrants for initially steam-preoxidized Zirlo
after 1000-h exposure in air at 300°C .......................................................................
22
Knoop hardness profiles for Zirlo in as-received condition and after several
treatments. .................................................................................................................
23
Parabolic rate constant in the pre-breakaway region for air oxidation of Zirlo,
indicating minimal effect of hydrogen up to ≈1000 wppm on the oxidation kinetics
at 300-600°C..............................................................................................................
27
Parabolic rate constant in the post-breakaway region for air oxidation of Zirlo,
indicating minimal effect of hydrogen up to ≈1000 wppm on the oxidation kinetics
at 400-600°C..............................................................................................................
27
Oxide growth rate constant in the pre-breakaway region for air oxidation of Zirlo,
indicating minimal effect of hydrogen up to ≈1000 wppm on the scaling kinetics at
400-600°C ..................................................................................................................
27
Oxide growth rate constant in the post-breakaway region for air oxidation of Zirlo,
indicating minimal effect of hydrogen up to ≈1000 wppm on the scaling kinetics at
400-600°C ..................................................................................................................
28
6.7
6.8
6.11
7.1
7.2
7.3
7.4
vii
Tables
3.1
Chemical composition of Zirlo used in the study.......................................................
5
4.1
Hydrogen concentration in Zirlo rings after prehydriding treatment in hydrogen
gas..............................................................................................................................
11
4.2
Oxide thickness and hydrogen concentration in Zirlo rings after steam exposure...
12
5.1
Air oxidation data for Zirlo capsules that were prehydrided in hydrogen gas for
170 h at 320°C ...........................................................................................................
15
Air oxidation results for 25-mm-long ring samples of Zirlo that were prehydrided
in hydrogen gas for 452 h at 320°C...........................................................................
16
Air oxidation results for Zirlo capsules that were prehydrided in hydrogen gas for
170 h at 320°C and steam preoxidized for 140 h at 550°C ......................................
19
Parabolic rate constants, based on weight change and oxide thickness, for air
oxidation of Zirlo cladding with various pretreatments ..............................................
26
5.2
6.1
7.1
viii
Executive Summary
The kinetics of cladding oxidation in an air environment is important in many safety-related
studies on nuclear reactors. For example, in a pressurized thermal shock event, the reactor
pressure vessel may be breached, leading to air intrusion in the core and consequent oxidation
of relatively cold fuel cladding. Another example is structural failure of the dry storage and
transportation cask, which may result in air intrusion and consequent interaction with the spent
fuel rods. In spent fuel pool accidents arising from loss of pool inventory, spent fuel rods can be
exposed to an air environment. In all these cases, knowledge of the air oxidation kinetics of the
cladding at relatively low temperatures is essential in assessing and determining the margin for
its integrity.
In an extensive experimental study, we previously tested Zircaloy-4, Zirlo, and M5
cladding materials to establish the air oxidation kinetics for the alloys in both bare and steampreoxidized conditions (Natesan and Soppet, 2004). Weight change and oxide thickness were
measured over a wide temperature range, and the results from that study were used to develop
oxidation rate correlations for all three materials. The study showed that all three alloys, initially
in the steam-preoxidized condition, could undergo further oxidation in the event of air ingress. It
was concluded that the rate at which the oxide scale grows is dependent on the cladding
temperature, exposure time, and alloy composition. The data and correlations on oxide scale
development were presented in an earlier report (NUREG/CR-6846).
The objective of this program is to obtain experimental data on the air oxidation kinetics of
unirradiated Zirlo cladding that has been prehydrided in hydrogen gas with or without steam
preoxidation. Experiments were conducted in which Zirlo ring and capsule specimens were
exposed in a hydrogen environment at 320°C and for various time periods. Prehydriding
treatment was tailored to obtain hydrogen concentrations in a range of 100-1000 wppm. Steam
preoxidation treatment was conducted to obtain an oxide thickness of ≈25 µm. The prehydrided
and prehydrided/steam-preoxidized Zirlo capsule specimens were subsequently oxidized in air
at temperatures in the range of 300-600°C. The maximum air oxidation times ranged between
≈1000 h at 300°C and 300 h at 600°C. Weight change and oxide thickness were measured on
the specimens exposed at various times to establish the kinetics of the scaling process as a
function of temperature.
Extensive metallography of cross sections of prehydrided and air-oxidized specimens was
conducted to measure the scale thickness and to establish the distribution of hydride
precipitates across the cladding thickness. Hardness measurements were made on specimens
with various treatments to examine the variation, if any, in the strength of the alloy due to
hydrogen ingress. Knoop hardness indentations that were made at several locations across the
cladding thickness in various specimens exhibited negligible variation, indicating uniform
distribution of hydrogen.
The air-oxidized specimens with various pretreatments were analyzed for hydrogen
content, and the results were used to correlate the parabolic oxidation rate and oxide-thickness
growth rate with hydrogen concentration in the alloy. Data indicated that exposure of the Zirlo
specimen at elevated temperature (e.g., steam preoxidation at 550°C) had a much more
softening effect than the hardening effect that can result from higher hydrogen content (e.g., 695
and 735 wppm in steam oxidized specimens).
ix
We concluded that hydrogen concentration in Zirlo in the range of 100-1000 wppm has
negligible deleterious effect on the kinetics of oxidation in air at temperatures in the range of
300-600°C. Furthermore, the scaling data indicated a negligible effect of hydrogen
concentration in the range of 100-1000 wppm on the scale growth.
x
Foreword
The kinetics of cladding oxidation in air environment is important in many safety-related
investigations. For example, in spent fuel pool accident arising from loss of pool water
inventory, spent fuel rods can be exposed to an air environment. Knowledge of air oxidation
kinetics of cladding at relatively low temperatures is needed in assessing safety and determining
the margin for clad integrity in all these cases. Prior data on cladding oxidation in air
environment was based on a very limited set of experiments not directly applicable to the low
temperature range of interest for the above cases. More recently, a research program,
sponsored by the NRC Office of Nuclear Regulatory Research was conducted at the Argonne
National Laboratory to obtain experimental data on air oxidation kinetics of cladding which are
representatives of current and/or projected inventory in operating reactors and in spent fuel
discharged after a medium or high level of fuel burnup. The program consisted of a large
number of experiments with specimens of Zircaloy-4, Zirlo, and M5 cladding in which the
specimens were subjected to isothermal air oxidation at different temperatures and for different
duration of time. The specimens were unirradiated but simulated in-reactor conditions in terms
of oxide growth. Results from these experiments, documented previously in a report
(NUREG/CR-6846) published in June 2004, indicated no discernible deleterious effect of the
pre-existing oxide layer from the in-reactor operation on further clad oxidation in an air
environment. The results also confirmed that the correlations developed in this experimental
study are in fair agreement with those used previously for the spent fuel pool risk study.
The present report documents the results of additional experiments conducted at the
Argonne National Laboratory to investigate the effect of hydrides on subsequent air oxidation
kinetics of cladding. The hydrides form on the cladding surface during in-reactor operation as a
result of hydrogen pickup from the corrosion process during oxidation of cladding by water and
steam. The hydrogen picked up migrates in colder regions of the cladding and tends to form
hydride rings or pockets of concentrated hydrides. The hydrides have potential deleterious
effects on subsequent oxidation kinetics and mechanical behavior of the cladding. The
additional experiments were performed with Zirlo cladding which comprises, in large part, the
inventory of most recently discharged fuel in spent fuel pool, particularly that related to higher
burnup fuel. The older fuels have Zircaloy-4 cladding, which is known to pick up substantially
more hydrogen than Zirlo at a higher burnup. However, these fuels have much less decay heat
associated with them and, as such, the cladding is not prone to significant thermal stress or
accelerated oxidation kinetics. In contrast to Zircaloy-4 and Zirlo, there is very little, if any,
current or projected inventory of M5 cladding in the spent fuel pool. Besides, the M5 cladding is
known to pick up less hydrogen during in-reactor operation compared to Zirlo and Zircaloy-4
and, as such, the effect of hydrides may be less significant for M5.
The results of the current series of experiments show that hydrogen pickup in Zirlo in the
range of 100-1000 wppm during in-reactor operation has negligible effect on the kinetics of
oxidation in air at temperatures in the range of 300-600°C. Noting that this level of hydrogen
pickup is representative of medium to high burnup operation, the results suggest no noticeable
deleterious effect of hydrides on air oxidation of Zirlo that was previously oxidized in steam
environment during in-reactor operation. At temperatures higher than 600•C, the hydrides go
into solution and do not affect the cladding behavior and thus, the air oxidation kinetics. The air
oxidation data reported here and in the companion report NUREG/CR-6846 will provide more
xi
realistic analysis of the spent fuel pool heatup and thus will add more confidence on the analysis
of spent fuel pool safety issues.
Farouk Eltawila, Director
Division of Systems Analysis and Regulatory Effectiveness
xii
Acknowledgments
This work is sponsored by the Office of Nuclear Regulatory Research, U.S. Nuclear
Regulatory Commission, under Job Code Y6512; Program Manager: S. Basu. Cladding tubes
of Zirlo were supplied by Westinghouse, Pittsburgh, PA. L. Cairo and M. Baquera assisted in
the metallography and in oxide thickness and hardness measurements of the hydrogen-charged
and air-oxidized specimens. Hydrogen, oxygen, and nitrogen analyses were performed by
Argonne National Laboratory, LECO Corporation, and Staveley Company.
xiii
xiv
1
Introduction
The kinetics of cladding oxidation in an air environment is important in many safety-related
studies on nuclear reactors. For example, in a pressurized thermal shock event, the reactor
pressure vessel may be breached, leading to air intrusion in the core and consequent oxidation
of relatively cold fuel cladding. Another example is structural failure of the dry storage and
transportation cask, which may result in air intrusion and consequent interaction with the spent
fuel rods. In both cases, knowledge of the air oxidation kinetics of the cladding at relatively low
temperatures is essential in assessing and determining the margin for its integrity.
Zirconium-based alloys are prone to oxidize fairly easily because of their affinity for
oxygen and the inherent thermodynamic stability of the zirconium oxide that forms when the
alloys are exposed to steam and air environments at elevated temperatures. High temperature
oxidation of zirconium and zirconium alloys in oxygen, air, and steam has been the subject of
extensive research due to their use as cladding materials in nuclear reactors (Cubiciotti, 1950;
Gulbransen and Andrew, 1957; Pemsler, 1962, 1964; Mackay, 1963; Wallwork et al., 1964;
Kidson, 1966; Com-Nougue et al., 1969; Pawel, 1979; Pawel and Campbell, 1980). Over the
years, several studies have been conducted to evaluate the kinetics of oxidation of Zr-based
alloys in steam environments, but most of the studies were conducted at temperatures >700°C
on bare alloys and for short time periods to predict the cladding behavior under loss-of-coolant
situations (Leistikow et al., 1978, 1980; Leistikow and Berg, 1987; Moalem and Olander, 1991;
Rosa and Smeltzer, 1980; Powers et al., 1994). The effect of hydrogen, if any, on the oxidation
process was not evaluated in these studies.
During the past two years, an extensive oxidation study was conducted at Argonne
National Laboratory on Zr-based cladding materials to establish the air oxidation kinetics for the
alloys in both bare and steam-preoxidized conditions (Natesan and Soppet, 2004). The
objective of the program was to obtain experimental data on the air oxidation kinetics of
unirradiated Zircaloy-4, Zirlo, and M5 cladding with an oxide layer that is representative of the
current inventory of spent fuel discharged after a medium or high level of fuel burnup. Weight
change and oxide thickness were determined for bare and steam preoxidized conditions over a
wide temperature range, and the results were used to develop oxidation rate correlations for all
three materials.
The expected oxide thickness on the cladding stored in spent fuel pool is in the range of
25-30 µm after service in medium burnup conditions and can be as high as 100 µm after high
burnup service. The aim of the earlier study was to preoxidize the bare cladding in a steam
environment to achieve an oxide thickness of 25-30 µm, which simulates the oxide layer on the
cladding in spent fuel pool (Natesan and Soppet, 2004). The air oxidation tests were performed
on the steam-preoxidized cladding at temperatures representative of cladding heatup in the
event of a partial or full draining of spent fuel pool coolant. Tests were performed on all three
alloys over a wide temperature range of 300-900°C, with emphasis on the low temperature
regime of 300-600°C.
The present study aims to examine the role, if any, of hydrogen ingress into the alloy on
subsequent oxidation of Zirlo cladding in an air environment. The report discusses the
hydrogen uptake by Zirlo in a gaseous hydrogen and a steam environment. Experimental
procedures are described and kinetic data are presented on air oxidation of prehydrided and
prehydrided/steam-preoxidized Zirlo alloy in both ring and capsule forms at temperatures in the
1
range of 300-600°C. Hydrogen concentration in the specimens in the prehydrided condition
was confined to a range of 100-1000 wppm. Extensive metallography of cross sections of
prehydrided and air-oxidized specimens was conducted to measure the scale thickness and to
establish the distribution of hydride precipitates across the cladding thickness. Hardness
measurements were made on specimens with various treatments to examine the indentation
size, and thereby establish the hydrogen distribution across the thickness of the cladding. The
air-oxidized specimens with various pretreatments were analyzed for hydrogen content, and the
results were used to correlate the parabolic oxidation rate and oxide-thickness growth rate with
hydrogen concentration in the alloy.
2
2
Background
Zirconium-based alloys, in general, have a strong affinity for oxygen, nitrogen, and
hydrogen, and the cladding, such as Zirlo, could develop a zirconium oxide (ZrO 2) scale during
reactor service, as a result of chemical reaction of Zr metal with water. At the same time, the
alloy also absorbs hydrogen, that is released from reaction of the Zr metal with water. The
thickness of the external oxide scale and amount of hydrogen ingress into the alloy are strongly
dependent on exposure time and temperature. Figure 2.1 shows the Zr-H phase diagram,
indicating a low solubility value for hydrogen in the alloy (α phase) at the reactor operating
temperature of 320°C (Hansen and Anderko, 1958).
Several investigators have measured the solubility of hydrogen in Zr-based alloys by a
variety of techniques (Gulbransen and Andrew, 1955; Sawatzky, 1960; Ostberg, 1962;
Westerman, 1966; Erickson and Hardie, 1964; and Kearns, 1967). Several studies concluded
that Zr and its alloys exhibit significant supersaturation of hydrogen, depending on the alloy
composition. As a result, there was significant scatter in the solubility values measured by
different investigators. Kearns (1967) measured solubility by a diffusion couple method, which
was not subject to supersaturation. To check possible compositional effects, he used iodide
and sponge zirconium, Zircaloy-2, and Zircaloy-4.
Kearns determined the terminal solubility by hydrogen analysis of the low-hydrogen part of
diffusion couples made by resistance welding hydride-bearing samples (2.5-cm long) to
opposite sides of hydrogen-free samples (0.64-cm long) and annealing to equilibrium in the
temperature range of 260 to 525°C. The hydrogen content of the high-hydrogen part of the
couple was in the range of 500-2000 wppm, which was sufficient to maintain a two-phase
mixture of hydride and saturated alpha solid solution during diffusion anneal. In this method,
supersaturation of the low-hydrogen part of the couple was avoided since diffusion raised the
level only to that of the alpha phase in the two-phase mixture. Hydrogen concentration in the
equilibrated specimens was analyzed by the hot vacuum extraction method. The best-fit
straight line through the data points for hydrogen solubility in annealed Zircaloy-2 and Zircaloy-4
was represented by an equation
Solubility (wppm) = 9.9 x 10 4 exp(-8250/RT)
(2.1)
where R is the universal gas constant (1.987 cal/mol·K), and T is absolute temperature (Kearns,
1967). A representation of the hydrogen solubility as a function of reciprocal temperature is
shown in Fig. 2.2. Hydrogen solubility values are 1, 71, 207, 460, and 851 wppm at 100, 300,
400, 500, and 600°C, respectively. It is believed that the solubility of hydrogen in Zircaloy-4 and
Zirlo is similar in the temperature range of the present study.
The hydrogen concentration range in the current air-oxidation study of Zirlo is
100-1000 wppm, which indicates that some hydrogen will be present as hydride precipitates
depending on temperature. By assuming that Zr hydride is a stoichiometric compound (ZrH2)
and the solubility of hydrogen is identical in Zircaloy-4 and Zirlo, we have calculated the total
hydrogen content in the alloy as a function of volume fraction of hydride precipitates at 100600°C. Figure 2.3 shows the total hydrogen content in the alloy as a function of hydride volume
fraction. The calculations indicate that for a total hydrogen content of 1000 wppm, the volume
fraction of hydride (at equilibrium) will be 0.055, 0.044, 0.03, and 0.005 at 100, 300, 500, and
600°C.
3
Figure 2.1. Zr-H phase diagram, indicating
extremely low value for hydrogen solubility in
Zr alloy (α phase) at the reactor operating
temperature of 320°C.
H
Temperature (°C)
600
400
200
300
100
Hydrogen concentration (wppm)
1000
Figure 2.2.
Solubility of hydrogen in
Zircaloy-4 as a function of reciprocal
temperature.
100
H (wppm) = 9.9 x 104 exp (-8250/RT)
10
1
10
15
20
25
(a)
2500
600°C
1200
550°C
Total hydrogen concentration (wppm)
Total hydrogen concentration (wppm)
!0,000/T(K)
500°C
300°C
200°C
2000
1500
100°C
1000
500
0
0
0.02
0.04
0.06
0.08
(b)
600°C
1000
550°C
800
500°C
600
300°C
400
200°C
200
100°C
0
0
0.1
0.002
0.004
0.006
0.008
0.01
0.012
ZrH volume fraction in Zr alloy
ZrH volume fraction in Zr alloy
2
2
Figure 2.3. Total hydrogen concentration as a function of volume fraction of ZrH2 phase at
temperatures in the range of 100-600°C: (a) 0-0.1 ZrH2 and (b) 0-0.01 ZrH 2.
4
3
Experimental Procedure
This section discusses the size and composition of the test material, type and geometry of
the specimens used in various tests, and the facilities and approach for prehydriding, steam
preoxidation, and air oxidation of the specimens.
3.1 Material
The Zirlo material used in the study was supplied by Westinghouse. The material,
supplied in the form of tubes, had dimensions of 0.374 ± 0.0020 in. OD (9.45 ± 0.05 mm), and
0.328 ± 0.0015 in. ID (8.33 ± 0.038 mm), with an average wall thickness of 0.023 in. (0.58 mm).
The chemical composition was determined by both Westinghouse and ANL, and the values are
listed in Table 3.1.
Table 3.1. Chemical composition of Zirlo used in the study
Element1
Westinghouse analysis
ANL analysis 2
Sn (wt.%)
0.99
0.73, 0.74
Fe (wt.%)
0.11
0.11, 0.12
Nb (wt.%)
0.98
0.96, 0.88
Cr (wt.%)
NR3
<0.01, <0.01
Si (wt.%)
0.005
<0.005, <0.005
Zr (wt.%)
Balance
Balance
Hf (wt ppm)
40
200, 200
C (wt ppm)
135
180, 90
O (wt ppm)
1100
1200, 1300
H (wt ppm)
NR
3.5, 5.7
N (wt ppm)
46
34, 36
Ni (wt ppm)
NR
<100, <100
Ta (wt ppm)
NR
<50, <50
W (wt ppm)
NR
<100, <100
1
Units of measure in parentheses. 2Duplicate analysis. 3NR = Not reported.
3.2 Specimen Geometry
We used two types of specimens: rings ≈6.5 mm long and capsules 75 mm long. The ring
specimens established the time for the prehydriding at 320°C to obtain the desired hydrogen
concentration in the cladding, and the time and temperature for steam preoxidation of capsule
specimens to obtain an oxide thickness ≈25-30 µm. Zirlo capsule specimens 75 mm in length
were fabricated, and the capsules were back filled with argon gas and welded shut in a glove
box. The capsule specimens were welded by e-beam on one side and tungsten inert gas (TIG)
welding on the other side. The end caps were fabricated from Zircadyne 702 material. These
specimens were used for air oxidation study in the initially prehydrided and prehydrided/steampreoxidized conditions.
3.3 Prehydriding of Ring and Capsule Specimens
Hydrogen charging of ring and capsule samples of Zirlo was performed in two similar
radiant-furnace systems, each equipped with a retort tube. Figure 3.1 shows a schematic of the
5
hydrogen-charging apparatus. Both systems are designed for experiments in hydrogen gas
with flow rates up to 300 cc/min at pressures from atmospheric (0.10 MPa) to 0.12 MPa. Flow
rates are computer controlled with Brooks mass-flow controllers. Furnace system-1 has a
quartz retort tube that is 24-in (60-cm) long with a 2 in. (5-cm) I.D. It also has a uniform heat
zone 6-in. (15-cm) long. System-2 has a 99.8% alumina retort tube that is 36-in (90-cm) long
with a 3.5 in. (8.75-cm) I.D. It has a uniform heat zone that is 8 in. (20 cm) long. Both systems
are able to maintain the temperature within 1°C of the desired value over the heat zone.
System-1 is capable of 320°C heat-up in 15 min and cool-down to room temperature in 2 h.
System-2 has a slower heat-up and cool-down due to its larger size. This condition also
minimizes the thermal shock fracture of the alumina reaction tube.
System-1 was used primarily to conduct kinetic studies of the hydrogen charging process
on ring specimens. System-2 was used for batch processing of Zirlo capsules for both
hydrogen charging and steam preoxidation.
Zirlo ring and capsule samples were prepared for hydrogen charging by a light polishing
with 1200-grit SiC paper to remove any surface oxides. They were then ultrasonically
degreased with acetone followed by drying with a warm air blower. Figure 3.2 shows typical
ring specimens after hydriding treatment at 320°C. Samples were weighed before and after
hydrogen charging with a Mettler Model M5 microgram balance that has five-decimal-place
precision. The difference yielded the hydrogen weight gain. Following weighing, the samples
were placed on a holder assembly and inserted into the uniform heat zone of the retort tube
such that the recording thermocouple well was at the midpoint of the holder assembly. The
holder assemblies in furnace systems 1 and 2 were constructed of quartz and Alloy 625,
respectively.
QUARTZ
SPECIMEN HOLDER
Oxygen
Filter
QUARTZ CHAMBER
FURNACE
ALUMINA
THERMOCOUPLE WELL
VENT GAS
TO
LAB EXHAUST
SYSTEM
UHP Argon
UHP Hydrogen
Mass Flow
Controllers
Back-Pressure
Regulator
Figure 3.1. Test facility used for hydrogen charging of ring and capsule specimens of Zirlo.
6
Figure 3.2. Typical ring specimens of
Zirlo after exposure for hydriding at
320°C.
After loading the samples into the furnace retort chamber, the port opening was sealed
closed with a Viton o-ring compression seal. The retort chamber was initially purged with ultrahigh-purity (UHP) argon Gas (99.999 vol.%) at 200 cc/min flow rate for a minimum of 4 h at
slightly above atmospheric pressure. Subsequently, the system was purged further by
switching to research-grade hydrogen (99.9995 vol.%) at the 100 cc/min flow rate for 20 h. The
hydrogen gas flowed through an oxygen absorbing gas-purifying filter (LabClear Model DGPR1) prior to entering the retort chamber. After the purging period, the furnace was heated to
320°C in 15 min for the quartz chamber and 3 h for the alumina chamber to begin the hydrogen
charging time.
Upon completion of the specimen exposure, the furnace was cooled down while hydrogen
gas flow was maintained at the test flow rate. The system-1 quartz chamber was cooled to
room temperature in ≈2 h, while the alumina chamber in system-2 required ≈4 h cool down
period. At room temperature, hydrogen gas was flushed out of the retort chambers with UHP
argon gas at a flow rate of 200 cc/min for 30 min. Furnace chambers were then vented, and
hydrogen-charged specimens were retrieved for visual inspection and weight-gain
determination. Samples were stored in polyethelene bags for future steam preoxidation and/or
air oxidation. Figure 3.3 shows capsule specimens of Zirlo before (left figure) and after
hydriding exposure for 170 h in hydrogen gas at 320°C (right figure).
Figure 3.3. Capsule specimens of Zirlo (75-mm long) before (in holder) and after hydriding
treatment. Dark gray color: as fabricated; dark black color: after 170-h exposure in hydrogen
gas at 320°C.
7
3.4 Steam Exposure of Prehydrided Ring and Capsule Specimens
Two tubular resistance-heated furnaces were used to expose the ring and capsule
specimens to steam oxidation. Figure 3.4 shows one of the facilities used for steam oxidation.
The system consists of a resistance-heated furnace with a constant temperature zone of
≈20 cm. The reaction chamber was made of high-purity alumina. The steam for the experiment
was generated by pumping distilled water from a water source and converting it to steam in the
preheat portion of the furnace, ahead of the specimen exposure location. The exhaust steam
from the chamber was condensed in a steam condenser. The flow rate was 6 cc/h of water. A
mass balance on the water flow showed that almost all the input water was collected as the
effluent, indicating that the steam consumption was negligible during the oxidation of
specimens. Argon gas was used to disperse the steam in the reaction chamber.
3.5 Air Exposure
Four resistance-heated furnaces were used for oxidation of prehydrided and
prehydrided/steam-preoxidized zirlo capsule specimens in air. The capsules were retrieved
periodically to measure the weight changes and determine the oxide thickness by optical
metallography.
Alumina chamber
Steam condenser
Furnace
Water source
Water pump
Controller
T recorder
Figure 3.4. Test facility used for steam preoxidation of prehydrided Zirlo specimens.
8
For the prehydrided/steam-preoxidized specimens that were subsequently exposed in air,
the specimens after each exposure time were cut, mounted, polished, and analyzed by
scanning electron microscopy (SEM) to determine the total oxide thickness and that developed
during the air exposure step. Total oxide thickness was measured in the four quadrants around
the capsule specimen, and the values were averaged to establish the scale thickness. Oxide
growth during air exposure was calculated from the measured weight change.
9
10
4
Prehydriding of Zirlo
4.1 Gas Phase Hydriding
Ring specimens of Zirlo were hydrided in 99.9995 vol.% hydrogen gas at 320°C, the
details of which were presented in Section 3.3. Table 4.1 lists the H concentration values
obtained on various ring specimens after exposure to hydrogen gas at 320°C. Figure 4.1 shows
the hydrogen concentrations obtained on several ring specimens as a function of exposure time.
Based on this information, conditions of 170 h at 320°C were selected for the exposure of Zirlo
capsule specimens to obtain a hydrogen concentration of 600 wppm, typical of levels
anticipated under high-burnup condition.
Table 4.1. Hydrogen concentration in Zirlo rings after prehydriding treatment in hydrogen gas
Specimen
Number
Exposure time in
hydrogen gas (h)
Measured
hydrogen
concentration
(wppm)
ZRL-1
-
3.5
As-received
CONAM
ZRL-2
-
5.7
As-received
CONAM
ZLH-A
6.3
90.6
None
Staveley
ZLH-B
12.4
92.7
None
Staveley
ZLH-C
28.4
175.0
None
Staveley
ZLH-D
68.4
321.0
None
Staveley
ZLH-D
68.4
360.0
None
Staveley
ZLH-E
64.2
202.0
None
Staveley
None
ANL
ZLH-F
24.2
1
187.0, 202.0
Specimen
Pretreatment
Hydrogen
Analysis
Performer
1
Duplicate analysis.
600
H concentration (wppm)
500
Figure 4.1.
Hydrogen
concentration in Zirlo ring
specimens as a function of
exposure time in pure
hydrogen gas at 320°C.
400
300
200
100
0
0
50
100
Exposure time (h)
150
200
11
4.2 Hydriding in Steam
We also exposed several ring specimens of Zirlo in a steam environment at 550°C for
several time periods. Our goal was to examine the hydrogen ingress into the material from
steam exposure and also to establish the oxide thickness for the exposed specimens. These
specimens were also analyzed for hydrogen content, and the oxide thickness was calculated
from the weight change. Figure 4.2 shows the hydrogen concentration and oxide thickness for
the Zirlo ring specimens as a function of exposure time in steam at 550°C. Based on these
data, prehydriding of bare capsules was followed by steam exposure to obtain 30-µm-thick
oxide. The hydrogen content of these specimens is expected to be higher than 600 wppm, and
the results from these exposures are discussed in a later section.
Table 4.2. Oxide thickness and hydrogen concentration in Zirlo rings after steam exposure
Specimen
Number
Exposure time
(h)
Weight
change
(mg/mm2)
AR
0
-2
Measured ZrO2
oxide thickness
(µm)
Measured
hydrogen
concentration
(wppm)
Average
hydrogen
concentration
(wppm)
-
5.0
5.0
1
ZRL-H
144.0
3.7 x 10
24.6
1265, 1156
1211
ZRL-J
139.5
3.9 x 10-2
27.0
1311, 12041
1258
-2
1
ZRL-K
48.0
1.1 x 10
10.8
223, 197
210
ZRL-L
15.9
5.5 x 10-3
4.7
20.3
20.3
ZRL-M
33.1
-3
9.5 x 10
8.3
122
122
ZRL-N
62.6
1.8 x 10-2
12.8
569
569
-2
ZRL-O
129.4
3.7 x 10
25.9
1252
1252
ZRL-P
178.4
5.2 x 10-2
38.6
1574
1574
254.9
-2
46.7
2221
2221
ZRL-R
7.2 x 10
1
2500
100
2000
80
1500
60
1000
40
500
20
0
0
0
50
100
150
200
250
Exposure time (h)
12
300
Oxide thickness (µm)
Hydrogen concentration (wppm)
Duplicate analysis; these were performed by LECO Corporation, and all others were performed at ANL.
Figure 4.2.
Hydrogen
concentration and ZrO 2
thickness in Zirlo ring
specimens as a function of
exposure time in steam at
550°C.
An attempt was made to correlate the oxide scale thickness with the hydrogen
concentration in the alloy and examine the role of dissolved oxygen in the water/steam. In our
steaming experiments, water (in the aerated condition) was pumped into the furnace and
converted into steam prior to reaction with the Zirlo specimens. As a result, there are two
possible reactions (extreme cases) that can lead to hydrogen ingress into the alloy:
Zr + 2H2O = ZrO2 + 4H (deaerated condition)
(4.1)
Zr + H2O + 1/2 O2 = ZrO2 + 2H (aerated condition)
(4.2)
Figure 4.3 shows a plot of measured and calculated hydrogen concentrations as a function of
oxide thickness for Zilro specimens after steam exposure. Also shown in the figure are the lines
that correspond to expected hydrogen in the alloy for oxide thicknesses based on reactions
(4.1) and (4.2).
Hydrogen concentration (wppm)
3500
3000
Deaerated
Zr + 2H O = ZrO + 4H
2
2500
2
2000
1500
Aerated
Zr + H O + 1/2O (dissolved) = ZrO + 2H
1000
2
2
2
Hydrogen Conc. with Calculated Oxide
500
Calculated Hydrogen Conc. (deaerated)
Calculated Hydrogen Conc. (aerated)
0
Hydrogen Conc. with Measured Oxide
0
10
20
30
40
50
60
70
Oxide thickness (µm)
Figure 4.3. Hydrogen concentration as a function of oxide thickness for Zilo specimens steam
oxidized at 550°C.
4.3 Hydriding of Capsules
Based on the H ingress data on ring specimens, we hydrided several 75-mm-long
capsules of Zirlo in a pure hydrogen gas at 320°C for 170 h. Figure 4.4 shows the weight gain
for 14 capsules from two exposure runs that were conducted to prehydride the Zirlo capsules.
The weight change for the capsules ranged between 6 x 10 -4 and 1.1 x 10-3 mg/mm 2. The
prehydrided capsules were used in subsequent tests for air oxidation with and without steam
preoxidation.
13
Weight Change (mg/mm2)
10-3
Figure 4.4. Weight change in 75mm-long capsule specimens after
170-h exposure in pure hydrogen
gas at 320°C.
10-4
1
2
3
4
5
6
7
8
9
10 11 12 13 14
Capsule Number
14
5
Air Oxidation of Prehydrided Zirlo
Prehydrided capsules 1-4 were selected for the air oxidation study at 300, 400, 500, and
600°C. The specimens were retrieved periodically to measure the weight change of the
capsules. The exposure times for these capsules ranged from a maximum of 1000 h at 300°C
to 300 h at 600°C. Tables 5.1 and 5.2 list the air oxidation data generated at various
temperatures and the measured concentrations for hydrogen, oxygen, and nitrogen in all the
specimens tested. Upon completion of the air exposures, the specimens were analyzed for
their hydrogen content by bulk analysis and for hydrogen distribution by metallography.
Figure 5.1 shows the weight change data obtained at 300-600°C during air oxidation of
Zirlo capsules, that were prehydrided for 170 h in hydrogen gas at 320°C. Also shown in the
figure are the air oxidation data developed earlier for bare Zirlo that contained ≈5 wppm
hydrogen (Natesan and Soppet, 2004). The results indicate that at hydrogen concentrations in
the range of 116 to 329 wppm (measured in these prehydrided capsules – see Table 5.1),
hydrogen has virtually no effect on air oxidation kinetics. Figure 5.2 shows the weight change
data obtained at 300-600°C during air oxidation of bare Zirlo capsules and Zirlo rings that were
prehydrided for 452 h in H2 gas at 320°C. Note that the sealed capsules of the bare alloy were
air oxidized on the OD side only, whereas the prehydrided ring specimens were oxidized on
both the OD and ID of the specimens. The data in Figs. 5.1 and 5.2 are normalized with respect
to area, and therefore, the weight change can be directly compared. Again, the rates are
comparable for both bare and prehydrided specimens at all temperatures except 600°C, where
the weight gain is slightly higher for prehydrided specimens.
Table 5.1. Air oxidation data for Zirlo capsules that were prehydrided in hydrogen gas for 170 h
at 320°C
Exposure
Specimen temperature
Number
in air (°C)
ZLC-38
ZLC-34
300
Exposure
time in
air (h)
120
310
621
1026
Measured
Weight
Calculated
hydrogen
change ZrO2 thickness concentration
(mg/mm2)
(µm)
(wppm)
170, 172
9.2 x 10-4
0.1
2.1 x 10-4
0.1
3.5 x 10-4
0.2
4.4 x 10-4
0.3
138, 130
Measured
oxygen
concentration
(wt.%)
0.144, 0.142
Measured
nitrogen
concentration
(wppm)
29, 26
ZLC-35
400
75
166
332
666
7.5 x 10-4
1.2 x 10-3
1.8 x 10-3
3.1 x 10-3
0.5
0.8
1.2
2.1
118, 116
0.194, 0.191
32, 32
ZLC-36
500
50
100
260
425
500
5.6 x 10-3
7.8 x 10-3
1.7 x 10-2
3.0 x 10-2
3.6 x 10-2
3.8
5.3
11.3
20.8
24.7
157, 156
0.848, 0.952
53, 54
ZLC-37
600
40
80
150
316
3.4 x 10-2
7.3 x 10-2
1.5 x 10-1
3.6 x 10-1
22.7
48.8
96.6
221.7
249, 329
4.45, 7.43
298, 300
15
Table 5.2. Air oxidation results for 25-mm-long ring samples of Zirlo that were
prehydrided in hydrogen gas for 452 h at 320°C
Specimen
Number
LHAE
Exposure
temperature
in air (°C)
300
LHAD
400
LHAK
500
LHAM
Calculated ZrO 2
thickness
(µm)
0.1
0.1
Measured
hydrogen
concentration
(wppm)
-
607
1.2 x 10-3
0.8
-
1010
-3
1.5
88
75
146
-4
5.5 x 10
1.0 x 10-3
0.4
0.7
-
297
1.4 x 10-3
1.0
-
609
-3
1.6
53.4, 53.5
50
101
-3
5.0 x 10
6.3 x 10-3
3.3
4.3
-
248
1.4 x 10-2
9.7
-
536
-2
21.4
101.8, 100
43
59
-2
4.3 x 10
6.1 x 10-2
29.5
41.9
-
80
8.4 x 10-2
57.9
150
-1
1.7 x 10
114.6
-
302
3.7 x 10-1
254.9
350.6, 360.2
2.2 x 10
2.3 x 10
3.1 x 10
closed symbols: bare alloy capsules
open symbols: prehydrided capsules for 170 h at 320°C
0.2
Weight change (mg/mm2)
600
Weight
change
(mg/mm2)
1.1 x 10-4
1.8 x 10-4
Exposure
time in air
(h)
100
295
Figure 5.1. Weight change during
air oxidation in capsule specimens
of bare and prehydrided Zirlo after
170-h exposure in pure hydrogen
gas at 320°C.
0.16
600°C
0.12
0.08
500°C
0.04
400°C
300°C
24
32
0
0
8
16
Square root of exposure time in air (h0.5)
16
closed symbols: bare alloy capsules
open symbols: prehydrided rings for 452 h at 320°C
Weight change (mg/mm2)
0.2
Figure 5.2. Weight change during
air oxidation in capsule specimens
of bare Zirlo specimens and in ring
specimens of prehydrided Zirlo
that were exposed for 452 h in
pure hydrogen gas at 320°C.
0.16
600°C
0.12
0.08
500°C
0.04
400°C
300°C
24
32
0
0
8
16
Square root of exposure time in air (h0.5)
Figures 5.3-5.5 show cross sections of Zirlo capsules that were prehydrided at 320°C,
prehydrided at 320°C/air oxidized for 500 h at 500°C, and prehydrided at 320°C/air oxidized for
316 h at 600°C. The prehydrided specimen containing ≈170 wppm hydrogen (which exceeds
the solubility value of 90 wppm at 320°C) exhibited small circumferential stringers of ZrH2
precipitate. However, these hydrides dissolved in the matrix (note absence of precipitates in
Figures 5.4 and 5.5) during air exposure at 500 and 600°C since the hydrogen solubility is much
higher than 90 wppm at these higher temperatures. The oxide thicknesses observed upon air
oxidation of the prehydrided specimens were similar to those observed during air oxidation of
bare Zirlo specimens, the results of which were reported extensively in an earlier report
(Natesan and Soppet, 2004). For example, the measured oxide thickness for the prehydrided
alloy was 25.6 µm (calculated value was 24.7 µm) after 500-h oxidation at 500°C, whereas the
corresponding value for bare alloy was 21.7 µm after 412-h oxidation at 500°C. Similarly, the
measured oxide thickness for the prehydrided alloy was 228.1 µm (calculated value was
221.7 µm) after 316-h oxidation at 600°C, whereas the corresponding value for bare alloy was
219.4 µm after 313-h oxidation at 600°C.
Figure 5.3. SEM photomicrograph of
cross section of Zirlo capsule prehydrided
for 170 h at 320°C.
17
Figure 5.4. SEM photomicrograph of cross Figure 5.5. SEM photomicrograph of cross
section of initially prehydrided Zirlo capsule, section of initially prehydrided Zirlo capsule,
after air oxidation for 500 h at 500°C.
after air oxidation for 316 h at 600°C.
18
6
Air Oxidation of Prehydrided and Steam-Preoxidized Zirlo
Several prehydrided capsule specimens were steam oxidized at 550°C for 140 h (a
condition that was used in earlier study) to develop ≈25-µm-thick oxide on the prehydrided
specimens. Two capsules were exposed in air at each of the four temperatures of 300, 400,
500, and 600°C. These specimens were retrieved periodically to measure their weight changes.
The exposure times for these capsules ranged from a maximum of 1000 h at 300°C to 300 h at
600°C. At each temperature, one capsule was retrieved at an intermediate exposure time and
destructively analyzed to establish the oxide thickness. Table 6.1 lists the air oxidation data
generated at various temperatures and the measured concentrations for hydrogen, oxygen, and
nitrogen in all the prehydrided/steam-preoxidized specimens tested. It is evident that
prehydriding/steam preoxidation results in hydrogen concentration of ≈733 wppm (see
specimen number ZLC-47 in Table 6.1). Air oxidation of these capsules at 300-500°C did not
significantly increase the hydrogen concentration. At 600°C and exposure times >150 h, where
significant cracking of the oxide scales is anticipated, the capsules showed somewhat larger
hydrogen concentration than the starting value of ≈733 wppm.
Figure 6.1 shows weight change data obtained at 500 and 600°C during air oxidation of
specimens that were initially either only steam oxidized or prehydrided/steam-oxidized to obtain
≈25-µm-thick oxide. The prehydriding condition was 170 h at 320°C. The steam oxidation
condition was 140 h at 550°C. The data indicate virtually no effect of prehydriding on air
oxidation at 500°C and slightly beneficial effect at 600°C. Figure 6.2 shows weight change data
Table 6.1. Air Oxidation results for Zirlo capsules that were prehydrided in hydrogen gas for
170 h at 320°C and steam preoxidized for 140 h at 550°C
Specimen
Number
Exposure
temperature
in air (°C)
Exposure
time in air
(h)
Weight
change in
air
(mg/mm2)
ZLC-47
-
-
-
ZLC-39
ZLC-41
ZLC-43
ZLC-46
1
300
400
500
600
Calculated
Measured
Measured
ZrO2
ZrO2 thickness in
hydrogen
thickness steam and in air concentration
(µm)
(µm)
(wppm)
-
23.7
Measured
oxygen
concentration
(wt.%)
Measured
nitrogen
concentration
(wppm)
729, 7371
1.00, 0.85 1
46, 571
96
-4
5.7 x 10
0.4
-
-
-
305
1.5 x 10-3
1.0
-
-
-
615
-3
2.0 x 10
1.4
973
2.3 x 10-3
1.6
73
1.9 x 10-3
160
2.8 x 10-3
326
-3
3.8 x 10
2.6
636
5.4 x 10-3
3.7
50
-3
3.8 x 10
100
7.6 x 10-3
262
-2
5.6 x 10
14.1
500
4.0 x 10-2
27.5
40
-2
4.2 x 10
27.9
80
8.7 x 10-2
56.4
150
-1
1.7 x 10
104.6
315
3.9 x 10-1
216.3
-
-
-
774, 779
1.07, 1.11
46, 45
1.3
-
-
-
1.9
-
-
-
27.9
739, 748
1.14, 1.15
43, 43
28.9
752, 757
1.09, 1.09
47, 47
2.6
-
-
-
5.2
-
-
-
37.2
752, 764
1.38, 1.45
75, 61
49.6
775, 784
1.91, 1.96
71, 67
50.1
726, 747
2.08, 1.61
84, 67
-
-
-
137.4
765, 881
5.05, 4.90
164, 163
269.2
849, 1136
8.85, 9.13
294, 298
25.8
Duplicate analysis.
19
open symbols: steam preoxidized
closed symbols: prehydrided and steam oxidized
Weight change (mg/mm2)
0.5
0.4
Figure 6.1. Weight change data
obtained at 500 and 600°C during
air oxidation of Zirlo capsule
specimens that were initially either
only
steam
oxidized
or
prehydrided/steam oxidized.
600°C
0.3
0.2
500°C
0.1
0
0
Weight change (mg/mm2)
0.016
5
10
15
20
25
30
Square root of exposure time in air (h 0.5)
35
40
open symbols: steam preoxidized
closed symbols: prehydrided, steam preoxidized
Figure 6.2. Weight change data
obtained at 300 and 400°C during
air oxidation of Zirlo capsule
specimens that were initially either
only
steam
oxidized
or
prehydrided/steam oxidized.
0.012
400°C
0.008
0.004
300°C
0
0
5
10
15
20
25
30
Square root of exposure time in air (h 0.5)
35
40
obtained in air oxidation at 300 and 400°C. Results show that, at 300°C for up to 1000 h, the
effect of prehydriding has virtually no effect on subsequent air oxidation. At 400°C, the
prehydriding seems to have a beneficial effect in reducing the oxidation rate during subsequent
air exposure. Figures 6.3 and 6.4 show the calculated oxide thickness for air-oxidized
specimens that were initially either only steam oxidized or prehydrided/steam oxidized.
Figures 6.5 and 6.6 show scanning electron microscopy (SEM) photomicrographs of cross
sections of a Zirlo specimen in the as-received condition and a prehydrided specimen that was
oxidized in air for 1026 h at 300°C. The latter specimen had an hydrogen concentration of
≈135 wppm, which exceeds the solubility value at 320°C. The microstructure clearly shows
precipitates of hydrides in the circumferential orientation across the entire cladding thickness,
indicating that once the hydrides are precipitated, subsequent oxidation occurs at low
temperatures such as 300°C has little effect on its distribution. Also shown in these figures are
Knoop hardness indentations that were made at several locations across the cladding
thickness. The size of the indentations within each specimen shows very little variation,
indicating negligible variation in hardness. Figures 6.7 and 6.8 show SEM photomicrographs of
cross sections of Zirlo specimens in steam-preoxidized/air-oxidized and prehydrided/steampreoxidized/air-oxidized conditions, respectively. Both these specimens were oxidized in air for
≈1000 h at 300°C. The measured hydrogen concentrations in these specimens were 695 and
20
300
600°C
Oxide thickness (µm)
250
200
open symbols:
steam preoxidized
closed symbols:
prehydrided, steam preoxidized
150
100
Figure 6.3. Oxide thickness data
obtained at 500 and 600°C during
air oxidation of Zirlo capsule
specimens that were initially either
only
steam
oxidized
or
prehydrided/steam oxidized.
500°C
50
0
0
5
10
15
20
25
30
35
0.5
Square root of exposure time (h )
Oxide thickness (µm)
8
6
open symbols:
steam preoxidized
closed symbols:
prehydrided, steam preoxidized
Figure 6.4. Oxide thickness data
obtained at 300 and 400°C during
air oxidation of Zirlo capsule
specimens that were initially either
only
steam
oxidized
or
prehydrided/steam oxidized.
400°C
4
300°C
2
0
0
5
10
15
20
25
30
35
0.5
Square root of exposure time (h )
776 wppm, respectively. Knoop hardness indentations that were made at several locations
across the cladding thickness in these specimens also exhibited negligible variation.
Figure 6.9 shows Knoop hardness profiles as a function of cladding thickness for an asreceived Zirlo specimen. Measurements were made in four quadrants of the tube, and they
indicate significant variation in hardness between the quadrants but also in the radial direction
within each quadrant. Figure 6.10 shows similar hardness profiles obtained for an initially
steam-preoxidized Zirlo after 1000-h exposure in air at 300°C. Steam preoxidation at 550°C for
140 h seems to homogenize the material with a resultant uniformity in hardness values in all
four quadrants and across the cladding wall. Figure 6.11 shows a composite plot of hardness
profile data developed for Zirlo with several pretreatments and air oxidation. The hydrogen
concentrations in these specimens ranged between 5 and 735 wppm. The data in Fig. 6.11
indicate that the exposure of the Zirlo specimen at elevated temperature (e.g., steam
preoxidation at 550°C) has a much more softening effect than the hardening effect that can
result from higher hydrogen content (e.g., 695 and 735 wppm in steam-oxidized specimens).
21
Figure 6.5. Knoop microhardness indentations Figure 6.6. Knoop microhardness indentations
on as-received Zirlo cladding tube.
on initially prehydrided Zirlo cladding tube after
1026-h air oxidation at 300°C.
350
350
300
300
Hardness Knoop number
Hardness Knoop number
Figure 6.7. Knoop microhardness indentations Figure 6.8. Knoop microhardness indentations
on initially steam-oxidized (for 140 h at 550°C) on initially prehydrided/steam-oxidized Zirlo
Zirlo cladding tube after 1000-h air oxidation at cladding tube after 973-h air oxidation at 300°C.
300°C.
250
200
150
quadrant 1
100
quadrant 2
quadrant 3
50
quadrant 4
0
250
200
150
quadrant 1
100
quadrant 2
quadrant 3
50
quadrant 4
0
0
0.1
0.2
0.3
0.4
Distance from OD surface (mm)
0.5
0
0.1
0.2
0.3
0.4
Distance from OD surface (mm)
0.5
Figure 6.9. Knoop hardness profiles for as- Figure 6.10. Knoop hardness profiles in four
received Zirlo in four quadrants.
quadrants for initially steam-preoxidized Zirlo
after 1000-h exposure in air at 300°C.
22
Hardness Knoop number
350
300
250
200
150
as received (5 wppm H)
prehydrided (42 wppm H)
100
prehydrided and air oxidized 1000 h at 300°C (135 wppm H)
steamed and air oxidized 101 h at 300°C (735 wppm H)
50
steamed and air oxidized 1000 h at 300°C (695 wppm H)
0
0
0.1
0.2
0.3
0.4
Distance from OD surface (mm)
0.5
Figure 6.11. Knoop hardness profiles for Zirlo in as-received
condition and after several treatments. Hydrogen concentrations
in different specimens are given in the legend.
23
24
7
Correlations for Oxidation Kinetics
In general, the kinetics of oxidation of Zirlo specimens were derived from weight change
data by plotting the weight change against square root of exposure time and fitting the data by
two lines to depict pre- and post-breakaway kinetics. The slopes of both fitted lines were
calculated and were used to determine rate constants for the oxidation process. The rates
developed using the weight change data at different temperatures were curve fitted in an
Arrhenius-type formalism,
Rate constant = A exp(-B/TK)
(7.1)
where A and B are constants, and TK is temperature in degrees Kelvin.
Equations for the air oxidation of steam-preoxidized Zirlo specimens in the lowtemperature range (Natesan and Soppet, 2004) are
2
4
2
4
Rate constant (kg /m ·s) = 0.07 exp(-14430/TK) for pre-breakaway, 573 ≤ TK ≤ 873
Rate constant (kg /m ·s) = 8.3 x 104 exp(-24000/TK) for post-breakaway, 673 ≤ TK ≤ 873
(7.2)
(7.3)
Equations for the air oxidation of bare Zirlo specimens in the low-temperature range are
2
Rate constant (in kg /m4·s) = 0.057 exp(-14240/TK) for pre-breakaway, 573 ≤ TK ≤ 873
(7.4)
2
Rate constant (in kg /m4·s) = 1.0 x 105 exp(-24580/TK) for post-breakaway, 673 ≤ TK ≤ 873
(7.5)
Correlations were also developed to depict the kinetics of oxide scale growth over the
temperature range of 300-600°C. Equations for oxide growth are as follows:
2
Oxide growth rate constant (µm /s) = 2662 exp(-12790/TK) for pre-breakaway, 573 ≤ TK ≤ 873
(7.6)
2
Oxide growth rate constant (µm /s) = 1.44 x 108 exp(-19610/TK) for post-breakaway, 673 ≤ TK ≤ 873
(7.7)
Since we acquired limited data on the air oxidation kinetics of prehydrided and
prehydrided/steam-preoxidized Zirlo specimens, an attempt was made to compare the data
from this program with extensive oxidation database developed earlier on bare and steampreoxidized Zirlo specimens (Natesan and Soppet, 2004). The parabolic rate constants and
oxide-growth rate constants from the current program are listed in Table 7.1, along with the data
obtained earlier on bare and steam-preoxidized Zirlo. Further, the data from this program were
used to evaluate, if any, the role of hydrogen content in the alloy in its behavior during air
exposure.
Figure 7.1 shows a plot of parabolic rate constant for oxidation of Zirlo in the prebreakaway region, as a function of hydrogen content in the alloy, irrespective of whether the
hydrogen ingress was achieved by prehydriding in hydrogen gas or during steam preoxidation.
A similar plot of parabolic rate constant for oxidation in the post-breakaway region is shown in
Fig. 7.2. The data indicate that rates may be somewhat lower for hydrogen contents of
25
Table 7.1. Parabolic rate constants, based on weight change and oxide thickness, for air
oxidation of Zirlo cladding with various pretreatments
Air oxidation
temperature
Initial condition
Steam preoxidized1
1
Bare
Prehydrided and
steam preoxidized
Oxide thickness rate constant
(µm2/s)
900
Prebreakaway
1.8 x 10 -6
Postbreakaway
5.7 x 10 -5
Prebreakaway
2.8 x 10 -2
800
3.1 x 10 -7
1.2 x 10 -5
1.9 x 10 -2
700
4.3 x 10
-8
-7
-3
2.0 x 10 -1
600
1.8 x 10 -8
1.9 x 10 -7
4.6 x 10 -3
9.0 x 10 -2
500
1.1 x 10 -10
8.2 x 10 -10
8.3 x 10 -5
8.4 x 10 -4
400
2.6 x 10 -11
4.6 x 10 -11
1.3 x 10 -5
2.6 x 10 -5
300
1.4 x 10 -12
-
5.5 x 10 -7
-
900
1.0 x 10
-6
2.4 x 10
-7
700
6.3 x 10
-8
600
1.1 x 10 -8
500
(°C)
6.9 x 10
11.3
6.3 x 10
-2
4.8
1.1 x 10
-2
2.7 x 10 -1
1.3 x 10 -7
3.9 x 10 -3
5.5 x 10 -2
1.9 x 10 -10
1.1 x 10 -9
1.2 x 10 -4
6.0 x 10 -4
400
3.5 x 10 -11
-
2.9 x 10 -6
-
300
-12
-
1.7 x 10
-7
-
3.5 x 10
-3
2.6 x 10 -2
-9
9.9 x 10
-6
7.2 x 10
-7
2.6
3.9 x 10
1.2 x 10
3.0 x 10
-5
5.1 x 10
Postbreakaway
3.8
-1
800
Prehydrided
Parabolic rate constant
(kg 2/m4·s)
600
9.1 x 10
500
1.9 x 10 -10
1.2 x 10 -9
5.7 x 10 -5
7.0 x 10 -4
400
4.1 x 10 -12
-
6.0 x 10 -6
-
300
-12
-
7.3 x 10
-7
-
6.8 x 10
-4
1.3 x 10 -2
1.3 x 10
1.4 x 10
-7
-8
600
1.1 x 10
500
6.9 x 10 -11
1.3 x 10 -9
6.9 x 10 -5
5.1 x 10 -4
400
1.3 x 10 -11
-
4.7 x 10 -6
-
300
-12
-
-7
-
1.5 x 10
1.9 x 10
-7
6.2 x 10
1
Values from earlier work reported in NUREG/CR-6846.
50-200 wppm at 400 and 500°C, but the data are not sufficient to quantify this effect. We can
state, based on the data in these figures, that the hydrogen concentration in Zirlo in the range of
100-1000 wppm has negligible deleterious effect on the kinetics of oxidation in air at
temperatures in the range of 300-600°C. Figures 7.3 and 7.4 show rate constants for oxide
growth as a function of hydrogen content in the alloy in the pre- and post-breakaway conditions,
respectively. These data indicate negligible effect of hydrogen concentration in the range of
100-1000 wppm on the scale growth.
26
600°C
Pre-breakaway
10-8
2
4
Parabolic Rate Constant (kg /m s)
10-7
Figure 7.1. Parabolic rate constant in the
pre-breakaway region for air oxidation of
Zirlo, indicating minimal effect of
hydrogen up to ≈1000 wppm on the
oxidation kinetics at 300-600°C.
10-9
500°C
10-10
400°C
10-11
300°C
10-12
10-13
1
10
100
Hydrogen concentration (wppm)
1000
600°C
Post-breakaway
10-7
2
4
Parabolic Rate Constant (kg /m s)
10-6
10-8
Figure 7.2. Parabolic rate constant in the
post-breakaway region for air oxidation of
Zirlo, indicating minimal effect of
hydrogen up to ≈1000 wppm on the
oxidation kinetics at 400-600°C.
500°C
10-9
400°C
10-10
10-11
10-12
10-13
1
10
100
Hydrogen concentration (wppm)
1000
10-2
600°C
2
Oxide rate constant (µm /s)
Pre-breakaway
10-3
500°C
10-4
10
Figure 7.3. Oxide growthc rate constant
in the pre-breakaway region for air
oxidation of Zirlo, indicating minimal effect
of hydrogen up to ≈1000 wppm on the
scaling kinetics at 400-600°C.
-5
400°C
10-6
300°C
10-7
10-8
1
10
100
Hydrogen concentration (wppm)
1000
27
100
Post-breakaway
2
Oxide rate constant (µm /s)
10-1
10
600°C
-2
Figure 7.4. Oxide growth rate constant in
the post-breakaway region for air
oxidation of Zirlo, indicating minimal effect
of hydrogen up to ≈1000 wppm on the
scaling kinetics at 400-600°C.
500°C
10-3
10-4
400°C
10-5
10-6
10-7
1
10
100
Hydrogen concentration (wppm)
1000
28
8
Conclusion
An extensive study was conducted on unirradiated Zirlo alloy to establish the air oxidation
kinetics in both prehydrided and prehydrided/steam-preoxidized conditions. Experimental data
were obtained on the air oxidation kinetics of Zirlo cladding that had been prehydrided in
hydrogen gas with or without steam preoxidation. Experiments were conducted in which Zirlo
ring and capsule specimens were exposed in a hydrogen environment at 320°C and for various
periods. Prehydriding treatment was tailored to obtain hydrogen concentrations in a range of
100-1000 wppm. Steam preoxidation treatment was conducted to obtain an oxide thickness of
≈25 µm. The prehydrided and prehydrided/steam-preoxidized Zirlo capsule specimens were
subsequently oxidized in air at temperatures in the range of 300-600°C. The maximum air
oxidation times ranged between ≈1000 h at 300°C and 300 h at 600°C. Weight-change and
oxide-thickness data were developed at temperatures in the range of 300-600°C. The results
were used to develop parabolic rate constants and oxide-growth rate constants. The following
is a summary of the information developed from the test program:
•
Hydrogen concentration in the specimens in the prehydrided condition was confined to a
range of 100-1000 wppm.
•
Extensive metallography of cross sections of prehydrided and air-oxidized specimens was
conducted to measure the scale thickness and to establish the distribution of hydride
precipitates across the cladding thickness.
•
Hardness was measured on specimens with various treatments to examine the variation, if
any, in the indentation size due to hydrogen ingress. Knoop hardness indentations were
made at several locations across the cladding thickness in various specimens and exhibited
negligible variation, which indicated an uniform distribution of hydrogen.
•
The air-oxidized specimens with various pretreatments were analyzed for hydrogen content,
and the results were used to correlate the parabolic oxidation rate and oxide-thickness
growth rate with hydrogen concentration in the alloy.
•
Data indicated that exposure of the Zirlo specimen at elevated temperature (e.g., steam
preoxidation at 550°C) had a much more softening effect than the hardening effect that can
result from higher hydrogen content (e.g., 695 and 735 wppm in steam-oxidized specimens).
•
It is concluded that hydrogen concentrations in Zirlo in the range of 100-1000 wppm have
negligible deleterious effect on the kinetics of oxidation in air at temperatures in the range of
300-600°C. Furthermore, the scaling data indicated negligible effect of hydrogen
concentration in the range of 100-1000 wppm on the scale growth.
29
30
References
Com-Nougue, J., K. Omo, B. de Gelas, G. Beranger, and P. Lacombe, 1969, J. Less-Common
Metals, 19, 259.
Cubiciotti, D., 1950, J. Am. Chem. Soc., 72, 4138.
Erickson, W. H. and D. Hardie, 1964, J. Nucl. Mat., 13, 254.
Gulbransen, E. A. and K. F. Andrew, 1957, Trans. Metall. Soc. AIME, 209, 394.
Gulbransen, E. A. and K. F. Andrew, 1955, Trans. AIME, 203, 136.
Hansen, M. and K. Anderko, 1958, Constitution of Binary Alloys, McGraw-Hill, 811.
Kearns, J. J., 1967, J. Nucl. Mat., 22, 292.
Kidson, G. V., 1966, Electrochem. Technol., 4, 193.
Leistikow, S. and H. v. Berg, 1987, “Investigation under Nuclear Safety Aspects of Zircaloy-4
Oxidation Kinetics at High Temperatures in Air,” 2nd Workshop of German and Polish Research
on High Temperature Corrosion of Metals, eds. W. J. Quadakkers, H. Schuster, and P. J. Ennis,
Julich, Germany.
Leistikow, S., R. Kraft, and E. Pott, 1980, “The Interaction between Corrosion and Mechanical
Stress at High Temperatures,” Proc. Eur. Symp., Petten, p. 123.
Leistikow, S., G. Schanz, and H. V. Berg, 1978, “Kinetik und Morphologie der Isothermen DamfOxidation von Zircaloy-4 bei 700-1300°C,” KfK 2587, Germany.
Mackay, T. L., 1963, Trans. Metall. Soc. AIME, 227, 1184.
Moalem, M. and D. R. Olander, 1991, “Oxidation of Zircaloy by Steam,” J. Nucl. Mater., 182,
170.
Natesan, K. and W. K. Soppet, 2004, “Air Oxidation Kinetics for Zr-Based Alloys,” Argonne
National Laboratory Report ANL/03-32, NUREG/CR-6846.
Ostberg, G., 1962, J. Nucl. Mat., 5, 208.
Pawel, R. E., 1979, J. Electrochem. Soc., 126, 1111.
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Powers, D. A., L. N. Kmetyk, and R. C. Schmidt, 1994, “A Review of the Technical Issues of Air
Ingression During Severe Reactor Accidents,” Sandia National Laboratories, Albuquerque, NM,
SAND94-0731.
Rosa, C. J., and W. W. Smeltzer, 1980, “The Oxidation of Zirconium in Oxygen/Nitrogen
Atmospheres,” Z. Metallkunde, p. 470.
Sawatzky, A., 1960, J. Nucl. Mat., 2, 62.
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Westerman, 1966, J. Nucl. Mat., 18, 31.
31
NRC FORM 335
(2–89)
NRCM 1102,
3201, 3202
U. S. NUCLEAR REGULATORY COMMISSION
BIBLIOGRAPHIC DATA SHEET
1. REPORT NUMBER
(Assigned by NRC. Add Vol., Supp., Rev.,
and Addendum Numbers, if any.)
(See instructions on the reverse)
NUREG/CR-6851
ANL-04/14
2. TITLE AND SUBTITLE
Hydrogen Effects on Air Oxidation of Zirlo Alloy
3.
DATE REPORT PUBLISHED
MONTH
YEAR
September
2004
4. FIN OR GRANT NUMBER
Y6512
5. AUTHOR(S)
6. TYPE OF REPORT
K. Natesan and W. K. Soppet
Technical
7. PERIOD COVERED (Inclusive Dates)
8. PERFORMING ORGANIZATION – NAME AND ADDRESS (If NRC, provide Division, Office or Region, U.S. Nuclear Regulatory Commission, and mailing address; if contractor,
provide name and mailing address.)
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439
9. SPONSORING ORGANIZATION – NAME AND ADDRESS (If NRC, type “Same as above”: if contractor, provide NRC Division, Office or Region, U.S. Nuclear Regulatory
Commission, and mailing address.)
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555–0001
10. SUPPLEMENTARY NOTES
S. Basu, NRC Project Manager
11. ABSTRACT (200 words or less)
An experimental program was conducted to generate data on the air oxidation kinetics of unirradiated Zirlo cladding with
preoxidation and prehydriding to simulate inventory of spent fuel discharge after a medium or high level of fuel burnup. The oxide
layer on the cladding was formed by a preoxidation step in a steam environment for 140 h at 550°C, which resulted in an oxide
thickness in the range of 25-30 µm. Prehydriding treatment was done by charging hydrogen to cladding and the process was
tailored to produce hydrogen concentration in the range of 100-1000 wppm, typical of medium to high burnup cladding. The
prehydrided and prehydrided/steam-preoxidized specimens were subsequently oxidized in air at temperatures in the range of 300600°C. The maximum air oxidation times ranged between 300 h at 600°C and ≈1000 h at 300°C. Weight-change and oxidethickness were measured on the specimens exposed at various times to establish the kinetics of the scaling process as a function
of temperature. Extensive metallography and hardness measurements were performed on the tested specimens to examine the
oxide scale development and hydrogen ingress into the material. Weight-change and oxide-thickness data, generated in the
present program, were used to develop correlations to depict the air oxidation behavior of prehydrided alloys with and without
steam preoxidation. A comparison of the oxidation data on Zirlo with and without prehydriding (performed in gas phase hydrogen
and/or in steam) indicated that hydrogen concentration in the range of 100-1000 wppm had minimal effect on the Zirlo oxidation
rate in air at temperatures in the range of 300-600°C.
12. KEY WORDS/DESCRIPTORS (List words or phrases that will assist researchers in locating this report.)
Air Oxidation
Hydrogen content
Oxide Thickness
Weight Change
Oxidation Correlations
Zirlo
13. AVAILABILITY STATEMENT
Unlimited
14. SECURITY CLASSIFICATION
(This Page)
Unclassified
(This Report)
Unclassified
15. NUMBER OF PAGES
16. PRICE
NRC FORM 335 (2–89)
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